This invention relates to apparatus for controlling the speed of an internal combustion engine on a truck, for example, when the engine is being used to drive a pump that supplies high pressure hydraulic fluid for operating hydraulic work cylinders, motors or other actuators for mechanisms on the truck.
The new speed control is especially useful for aerial bucket trucks that are sometimes called cherry pickers. Such trucks are typically provided with articulated booms, or an extending stinger or stingers that support a bucket or buckets in which a person or persons stand while working on street lights, telephone and electric lines and doing painting work, for example. Typically, hydraulic actuators such as work cylinders are used to drive the components of the boom assembly in and out, up and down and left and right in proximity with the work. Comfort, convenience and safety of the worker requires that he be able to position the bucket precisely with respect to the work and to obtain an immediate movement response when one or more controls that are mounted in the bucket or in the area of the bucket are operated in contemplation of moving or repositioning the bucket.
The internal combustion engine that propels the truck is used for driving a pump that provides the hydraulic pressure for extending and contracting the work cylinders which act on the boom sections to move the bucket. In providing this hydraulic power, it is desirable to have the engine running at the lowest recommended idle speed and for the pump to be producing minimum volume and pressure until the operator uses a control to effect a bucket shift in one direction or another. Then, to provide fast response of the hydraulic mechanism, it is desirable to have the engine speed rise from idle speed to a predetermined maximum as rapidly as possible so there will be a corresponding rapid increase in pump volume and pressure.
A common problem in prior art throttle control systems is the delay that occurs between the time the operator manipulates a control valve handle and the time adequate pressure builds up to cause the bucket movement. This makes it difficult for the operator to jog or feather the bucket movement into the desired location and causes undesirable anticipation by the operator of bucket movement following control valve actuation. The lack of positive and fast response also decreases operational efficiency and safety and increases engine fuel consumption.
Various types of hydraulic throttle controls for pump driving engines are in use. Typically, there is a conduit leading from the output of the pump to one or more series or parallel connected work cylinder control valves which allow the hydraulic fluid from the pump to return to a sump when a control valve is not actuated. Low pump output pressure exists in the conduit at this time and the throttle on the driving engine senses this low pressure and causes the engine to run at idle speed. When a valve is actuated to operate a work cylinder, the return flow to the sump is cut off, and the pressure rises at the output of the pump. A desirable feature of any system would be to have the throttle control sense the increase in pressure and immediately switch the engine to run at a higher speed to thereby increase pump output and cause immediate movement of the work cylinders. Typically, in prior art controls, pressure in the conduit leading from the pump output to the control valve is sensed with a cylinder and piston arrangement. The cylinder has an input from the pump output conduit. The piston is moved in one direction or another in response to increases and decreases in pressure. The piston works against a spring and is connected by way of suitable linkage to the engine carburetor throttle. A pressure increase in the pump output conduit, due to the fluid return being stopped by actuation of a control valve, moves the piston in a direction to open the throttle and increase engine speed. A pressure decrease in the conduit causes the piston to move in a direction that causes the throttle to return the engine to idle speed. A disadvantage of this system is that it takes time to drive the pressure sensing piston to the end of its travel to obtain the higher engine speed. In prior art systems there is as much as a ten second lag before the engine reaches proper speed to cause the pump to build up maximum hydraulic pressure. In order to improve the response rate, the engine idle speed adjustments are customarily set high or fast. This tends to reduce the differential between idle speed pressure and work cylinder operating pressure. Typically, the engine might have to operate at an idle speed of 700 or 800 rpm compared to a normal lower and more desirable idle speed of 550-600 rpm, for example. However, the higher idle speed does not reduce the delay to a desired minimum. The higher idle speed results in increased engine fuel consumption and maintenance. High idle speed also increases ambient noise levels, an undesirabe factor when oral communication between personnel is necessary.
Also, in the operation of a boom truck with hydraulic devices, the hydraulic system is called on to actuate the devices only a small fraction of the working time. In other words, the driving engine is in the idle mode most of the time. This factor accentuates the undesirability of high idle speeds.
In prior art throttle control systems which use a hydraulic piston for regulation of engine speed there is also a substantial delay before the engine returns to idle speed after the hydraulic actuator control is returned to the neutral position. In part, a reason for this is that even though return of the pump output to the sump is being allowed, there is always some back pressure in the system, due to fluid friction encountered in tubing, hoses, fittings, and system filter or filters. The system back pressure also varies greatly due to temperature changes which change the viscosity of the hydraulic oil. Higher viscosity associated with lower temperatures results in a delay in the fall-off of the system pressure, prolonging the higher engine speed interval and contributing to increased fuel consumption and maintenance.
A hydraulic pressure switch and an electroresponsive means has been tried and used but has most of the problems mentioned above because a pressure switch has an inherent differential, that is, the pressure required to close its contacts is higher than the pressure required to open its contacts.